Update common.json

schema/common.json

 {
   "$schema": "http://json-schema.org/schema#",
-  "id": "https://uri.etsi.org/ngsi-ld/schema/common.json",
+  "id": "https://forge.etsi.org/gitlab/NGSI-LD/NGSI-LD/raw/master/schema/common.json",
   "title": "NGSI-LD Common Definitions",
   "description": "NGSI-LD Common",
   "definitions": {